Progress and Promise in Brain Tumor Research

Podcast Transcript

Intro: Neuro Pathways, a Cleveland Clinic podcast, exploring the latest research discoveries and clinical advances in the fields of neurology, neurosurgery, neuro rehab, and psychiatry.

Glen Stevens, DO, PhD: Despite decades of research, brain tumors remain among the deadliest form of cancer. Approximately 700,000 Americans are living with a primary brain tumor and an estimated 10% will receive a brain tumor diagnosis in 2021. Although brain tumors have proved challenging to treat, in recent years, significant advances have come to light that may give hope for enhanced survival. In today's episodes of Neuro Pathways, we're discussing progress and promise in brain tumor research.

I'm your host, Glen Stevens, neurologist, neuro-oncologist in Cleveland Clinic's Neurological Institute. I'm very pleased to have Dr. Justin Lathia join me today for today's conversation. Dr. Lathia is the co-director of Cleveland Clinic's Lerner Research Institute Center of Excellence in brain tumor research and therapeutic development, and co-leader of the molecular oncology program in the Case Comprehensive Cancer Center. Justin, welcome to Neuro Pathways.

Justin Lathia, PhD: Glen, thank you so much for the invitation to participate, and I really look forward to our discussion.

Glen Stevens, DO, PhD: So to start with, when we talk about glioblastoma, the most common primary malignant brain tumor of adults with a mean life expectancy of 15 to 18 months, the standard treatment unfortunately has not changed much since 2005 other than the addition of tumor treating fields. No new drugs have been approved in the last decade. The last approved drug was Avastin or bevacizumab in about 2017 for recurrent glioblastoma. Can you set the stage for us and provide insights on where brain tumor research started and where we're headed?

Justin Lathia, PhD: I think the one thing to realize is brain tumors are far more complex, I would argue, than a lot of other diseases. And as a clinician, you know this Glen, that brain tumors seem to be a graveyard for successful therapies in other cancers. You brought up bevacizumab, which is an antiangiogenic. And I remember starting my career in brain tumor research in the late 2000s and having angiogenesis be really a cornerstone of the field and to watch that fail and then wants a series of targeted therapies fail. And now in the advent of immunotherapy, there's been multiple phase two and phase three clinical trials that are not reporting any significant increase in survival through a variety of different inhibitors, whether it's immune checkpoint inhibitors or various kinds of vaccines. So what's been interesting is you're faced with that clinically, but I would also argue that there's been an investment, scientifically, made in brain tumors.

Brain tumors seem to be one of the first tumor types that are extensively characterized. So what do I mean by that? So The Cancer Genome Atlas was an NIH sponsored project. And what was the first tumor, or one of the first tumors, that they characterized? It was glioblastoma. So I think that there's been a push from the community to better understand the molecular genetics of glioblastoma and that's gone even further. So we're now in the advent of single-cell profiling. So single-cell RNA sequencing, and again, the first few papers that were coming out in this field were all glioblastoma driven. So in terms of where the field stands, I think we've been the beneficiary of a lot of high resolution characterization and description. And I think we're now at the precipice on leveraging that because what these high resolution approaches have shown us is more layers of complexity within brain tumors. And I think the community is actively integrating those new data sets to understand new molecular targets.

Glen Stevens, DO, PhD: So just a fun fact to throw out since it was discussed a little bit with antiangiogenesis. Judah Folkman, who is described as the father of antiangiogenesis, is originally from Cleveland and born here. And he published his paper, The Angiogenic Switch Process, that really defined anti-angiogenesis in 1971. And here we are in 2021, I mean, it's a remarkable period of time that you would have liked to have seen a better movement forward in the treatment of glioblastoma. So moving forward, although immunotherapy, as you alluded to, is revolutionizing cancers and certainly in the lay public with President Carter and his melanoma and the use of immunotherapies has really gained a lot of prominence in the literature over time. Its use in glioblastoma, as you mentioned, has lagged behind in this process. Why is that? Why is it that the immunotherapies aren't working in the brain?

Justin Lathia, PhD: So I alluded to previously that brain tumors, and let's use glioblastomas as a case study for this, it's just far more complex. So there's a variety of studies, some are clinical, some are preclinical, some are basic mechanistic, but they're all pointing to the same fact that there's two things that are happening in glioblastoma at higher rates than compared to other tumors. One is the change in the tumor micro environment. So I think we have an appreciation that, within the solid tumor, there's actually non-tumor cells. And there are reports that up to 30% or more of the actual cells in a glioblastoma aren't tumor cells, they're actually cells of the myeloid lineage. So what you have is a process where you have an increase and an influx in the myeloid lineage. So these can come from bone marrow derived cells or the resident myeloid population in the brain, which are microglia.

And what ends up happening is there is a two-way conversation that goes on between the tumor cell and the myeloid cell. The myeloid cells end up becoming what we like to call as pro-tumorigenic, so they actually support growth. They either directly, or by direction of the tumor cells, secrete pro-tumorigenic cytokines and chemokines. So the tumor itself is coaxing its environment to drive growth. So you have that on the one hand. On the other hand, if you look at the lymphoid population, which you would hope would actually detect, attack the tumor cells, they're sequestered in the bone marrow. So it's almost a perfect storm. You're dramatically increasing local immune suppression and generating a population of myeloid cells that drive tumor growth. And at the same time, you're decreasing T-cell access activity in the tumor. And I think that that's really at the crux of why these therapies are failing.

So again, looking at immune checkpoint inhibitors. You can activate T-cells and they got to get into the brain and stay into the brain. But if they're coming into an environment that is already highly suppressive, they're not able to overcome that. It's a poor analogy, but you can think about driving. If you're going to drive, you got to take your foot off the brake and also apply the gas. And what glioblastoma is doing is preventing both. It's a heavy foot on the break though the suppression, and it's not allowing the gas pedal to be pushed.

Glen Stevens, DO, PhD: Very well explained. So specific to glioblastoma, and you just barely brushed on this previously, but can you discuss single-cell RNA sequencing and how it's providing additional insights into brain tumor research?

Justin Lathia, PhD: Yeah. So I think this has been a technology that has given us a lot of insight into what we call the cellular heterogeneity. We've always had an appreciation. This is going back over a hundred years with rudimentary pathology looking at a tumor, and basically identifying different types of cells. So what single-cell profiling is allowing us to do is to understand the extent of this heterogeneity, and it's really helping us stitch together the whole picture. And there's some really fancy bioinformatics that can be applied to it that'll allow us to predict things like mutational trajectories or lineage relationships, and really put the pieces of plasticity together. There's a collection of papers that came out in nature cancer in February 2021 that all went at this a little bit differently. And it's a really beautiful set of three papers.

We were asked to write the news and views. So if people are interested, they can take a look at that. But there, we put the three papers together and what the single-cell sequencing has shown is that there's differences in programs. Some tumors are relying on wound healing genetic programs, some on developmental programs, there are differences in metabolism. So some tumors are relying on different metabolic pathways versus others. And this is being traced down to the single-cell level. So I think with this higher resolution, it's actually allowing molecular targets to be identified. And what's actually very promising is, my hypothesis or my speculation is we have so many drugs that target so many pathways and very few of them were even developed or thought of for brain tumors, so we have a whole toolbox. Once we know the pathways, the programs, and the networks that need to be targeted, I think we have an opportunity.

And I could really quickly put a plug in here for the work being done by the multiple sclerosis team at Cleveland Clinic, right? So they identified a drug that was brain penetrating, that they were involved in a phase two clinical trial. And we actually found that that same drug, so it targets a couple of pathways, but one is called macrophage migration inhibitory factor. So it turns out that they had done a lot of the work on that drug. We were able to quickly use that drug and look at it in our animal models, and it seemed like it was giving us a signal. So we're in the process now of trying to think about a clinical trial based on that. So this is really circling back to the fact that the more information we know about these tumors, it gives us additional molecular targets.

Glen Stevens, DO, PhD: Well, it's all very exciting, and I love to hear the collaboration and the multiple papers coming out together. This is really where we need to move forward, no question about it. So again, you've discussed this a little bit. I'm not sure if you want to add to it, but I know, and congratulations, I know you secured a grant that involved immune evasion strategies for glioblastoma. Do you want to share anything else about the research on that? Is this to do with your myeloid derived?

Justin Lathia, PhD: Yeah. Glen, thank you for the prompt. Let's take a deeper dive. Work had been done here actually by some of our former colleagues, including Mike Vogelbaum, MD, PhD, who really showed that, in human patients, these immature cells called myeloid derived suppressor cells were actually highly enriched in the circulation of glioblastoma patients. So working with Mike over the last five years, and Mike is actually the co-principal investigator on this grant you were mentioning. Between our two groups, we've been really hitting this at multiple angles. So one is, we've done more characterization in a variety of brain tumor patients who you guys have treated clinically. So we've looked in their blood and it turns out that these myeloid derived suppressor cells are actually enriched in glioblastoma patients compared to lower grade patients. And then when we subdivide glioblastoma patients that do worse versus better, the patients that do worse actually have a higher amount of these myeloid derived suppressor cells in the circulation and in the tumor micro environment.

So what do these cells do? We still actually don't know, there's a bit of a controversy in the field. We know they're immature, we know they go to sites of injury, or in this case, a tumor, and then they can suppress the immune system in a variety of ways. So we've been doing some work to try to target them. So for example, we just finished a phase zero, phase one that was led by David Peereboom, MD, who one of your colleagues, where we actually tried to target them in recurrent glioblastoma patients with low dose chemotherapy. And it seemed like we were able to reduce the number of cells. And that provides a very important proof of principle. So we've taken this back into the lab and here's several questions we're asking, right? One is what drives these? And one of the signals that drives these is macrophage migration inhibitory factor like I discussed.

So that's really the central part of that grant, is what is the role of macrophage migration inhibitory factor? And we're looking at this in a variety of ways. The first is, what receptors are on the myeloid derived suppressor cells that engage macrophage migration inhibitory factor and promoted signaling? Now, the other thing I should tell you, and this adds to the complexity, is myeloid derived suppressor cells come in two subsets. For simplicity's sake, I'll refer to them as the monocytic and the granulocytic. So we're really trying to understand what the difference between these subsets are. And we had a recent paper where we showed that the monocytics are more enriched in the tumor micro environment, and they actually express an MIF receptor called CD74, and that can be targeted with the drug I was telling you about.

The granulocytic subset is more enriched in the circulation. Now, it gets more interesting because the monocytic subset can actually be targeted as well by chemotherapies. And the granulocytic subset can be targeted, for example, by antagonizing IL-1beta. Now here's the final twist, there's actually a sex difference. So it turns out that males have more monocytic myeloid derived suppressor cells in general, and females have more granulocytic. So those experiments I was telling you about with chemotherapy and antagonizing IL-1beta. In our animal models, they only worked in one sex. So the chemotherapy only worked in recipient mice that were male and the anti IL-1beta only worked in the recipient mice that were female.

Glen Stevens, DO, PhD: Justin, this tags into really the next question that I was going to ask you, and maybe you've said all you want to say about it, but let's go back to the sex differences. Tell me a little bit about the difference between males and females that have glioblastoma and what you've learned.

Justin Lathia, PhD: It's an interesting concept that has really been staring us in the face in the field for four years. And Glen, I know you actually see this on a daily basis as someone who treats patients. So let's start epidemiologically. So epidemiologically, males get glioblastoma one point six times more frequently than females. Now, if you adjust for all clinically relevant variables, and there was a study that was led by Jill Barnholtz-Sloan from Case Western who's a colleague of ours through the Case Comprehensive Cancer Center, she was able to show that males actually do worse than females when you take into account all clinically relevant variables. Now that really got us thinking, is there a deeper level? So in collaboration with the group at Wash U, we looked at the molecular genetics of this. And this paper was published, I think in 2019. So we were able to identify with this group that there were male and female specific molecular alterations.

So there were differences in mutations between male and female tumors. And this led to a variety of different things, including differential drug sensitivity. So we did a large scale screen and we identified inhibitors that were more effective in males or females. And it's in the supplemental data in that paper, but it was shocking to us that temozolomide, standard of care for glioblastoma, is actually more effective against female tumor cells than male tumor cells. So now there's a difference epidemiologically and at the molecular genetic level. There's a difference in immune response.

So it's long been appreciated that females have a more active immune system. And this may explain why they're more likely to have autoimmune diseases, whereas males are more likely to get cancer. It was initially hypothesized this was just due to bad behavior and poor lifestyle choices. But again, when you normalize all of that, you have males getting cancer more frequently in general than females, females getting more auto immune disease. So now if you take a deeper dive and integrate what I just told you about myeloid-derived suppressor cells, it all starts to make sense. I think the immune response itself is actually different in a male or a female. And what we're now looking at in the lab very actively is, are there differences in male and female microglia? Are there differences in male and female pre-clinical models in the context of immunotherapies? So that's really where we're sitting right now.

Glen Stevens, DO, PhD: So as a one-off, any significance of BMI in this relationship?

Justin Lathia, PhD: Yeah. Yeah. So we've been thinking a lot as well about obesity and cancer. So, there's a huge link between obesity and cancer. Again, endometrial cancer, breast cancer, liver cancer. These are all examples of where there's quite a strong link between obesity and cancer. So, we're just finishing a study now. It's unpublished, but I'll tell you the broad strokes. And if you're interested, the paper is on the bio-archive preprint server. And what we were able to find, because it's actually been shown in meta-analyses, mainly in females, that obese females don't do as well in terms of glioblastoma progression.

So in our mouse models, we actually put female mice on a high fat diet, and we found that their tumors were more aggressive. And what was happening is there was a unique tumor suppressor that we came across called hydrogen sulfide. Turns out, hydrogen sulfide was even more suppressed with the high fat diet. And if we restored hydrogen sulfide levels, we did this pharmacologically, we actually could kill the tumor cells and reduce tumor growth. So we're just scratching the surface of the link here between obesity and glioblastoma. And you know, it begs the question about, are there dietary intervention opportunities? And that that's gets me thinking about some patients who are adopting or trying to adopt a ketogenic diet. So I think this is all linked.

Glen Stevens, DO, PhD: So just to take it another step, what about the microbiome in glioblastoma? Do you want to go there?

Justin Lathia, PhD: I don't think we have a choice but to go there. And the reason why is we're very lucky to have a very good microbiome center here. It's actually in the same department where my laboratory is, and we've collaborating with them quite a bit. So, I'll give you some examples. We've done some studies in mice where we know that the bacterial species in the guts of the mice that have tumors are different than the sham controls. We actually have been able to identify male- and female-specific strains. We actually, in our pre-clinical models, it's early days now, but we can transplant into germ-free mice, and those tumors seem to be more aggressive. And we're finally rounding this out by profiling both mice and human blood samples to look at metabolomics and metabolic pathways that are micro derived. So it turns out that once you eat food, it's obviously digested and the bacteria in your gut actually help drive metabolic conversion of unique molecules. So we're thinking about those in terms of not only biomarkers, but potentially molecular targets.

Glen Stevens, DO, PhD: Well, this is a fantastic meandering through brain tumor research. Anything else exciting on the horizon that we haven't discussed that you'd like to mention?

Justin Lathia, PhD: The thing I would leave everyone with is that glioblastoma is an extremely challenging disease, but I think that there's a lot of unique opportunities. And this spans at multiple levels, right? Thinking about the disease as a sex-specific disease, how are males and females different? Thinking about how that's integrated into clinical trials I think is going to be a great step. I also think harnessing a lot of the new knowledge, whether it's single-cell sequencing or exploding understanding of the microbiome and how it modulates disease is going to provide opportunities to develop the next generation of therapeutic interventions.

Glen Stevens, DO, PhD: Well, Justin, thank you very much for joining us today. This has been very insightful and I appreciate your time.

Justin Lathia, PhD: Thanks for the opportunity, Glen.

Outro: This concludes this episode of Neuro Pathways. You can find additional podcast episodes on our website, ClevelandClinic.org/neuropodcast or subscribe to the podcast on iTunes, Google Play, Spotify, or wherever you get your podcasts. And don't forget you can access real-time updates from experts in Cleveland Clinic's Neurological Institute on our consult QD website. That's consultqd.clevelandclinic.org/neuro or follow us on Twitter @CleClinicMD, all one word and thank you for listening.